Solid-state image sensor and image pickup apparatus including the same

ABSTRACT

A solid-state image sensor includes pixels each including first and second photoelectric conversion portions provided in a substrate, the second photoelectric conversion portion having lower sensitivity than the first photoelectric conversion portion; a barrier region provided between the first and second photoelectric conversion portions; a waveguide provided on a light-entrance side of the substrate and including a core and a cladding; and a protective layer provided between the waveguide and the substrate. Seen in a direction perpendicular to the substrate, a center of an exit face of the core is on a first-photoelectric-conversion-portion side with respect to a center of the barrier region in each pixel in a central part of a pixel area. A standard deviation of a refractive-index distribution of the protective layer in a region directly below the exit face of the core is 0.1 or smaller in an in-plane direction of the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to solid-state image sensors andparticularly to a solid-state image sensor included in an image pickupapparatus such as a digital camera.

2. Description of the Related Art

In recent years, there has been an increasing demand for images taken byapparatuses, such as a digital camera, with wider dynamic ranges. Inresponse to such a demand, a solid-state image sensor is proposed byJapanese Patent Laid-open No. 2004-363193, in which a plurality ofphotoelectric conversion portions having different areas are provided ineach pixel, so that two kinds of pixel signals, which are ahigh-sensitivity signal and a low-sensitivity signal, are acquired. Witha combination of the two signals, the dynamic range is widened.

There has been another increasing demand for a camera capable of takinga moving image and a still image simultaneously. In general, to acquirea smooth moving image, the moving image may be taken with a period ofexposure time that substantially corresponds to the frame rate of thereading by a solid-state image sensor. In contrast, to take a stillimage, the period of exposure time may be set in accordance with thespeed at which the object moves. Hence, to take a still image and amoving image simultaneously, two kinds of pixel signals based ondifferent periods of exposure time need to be acquired.

In Japanese Patent Laid-Open No. 2004-120391, a solid-state image sensoris disclosed that includes a plurality of photoelectric conversionelements (equivalent to the photoelectric conversion portions accordingto Japanese Patent Laid-Open No. 2004-363193) provided in each pixel andbeing based on different periods of exposure time so that a moving-imagesignal and a still-image signal can be acquired simultaneously.Photoelectric conversion elements for a relatively short period ofexposure time each have a relatively large area, whereas photoelectricconversion elements for a relatively long period of exposure time eachhave a relatively small area. Furthermore, the sensitivity ofphotoelectric conversion elements for a moving image is different fromthe sensitivity of photoelectric conversion elements for a still image.

Note that “sensitivity of a photoelectric conversion portion” is definedby the ratio of the amount of charge accumulated in the photoelectricconversion portion to the quantity of light that is incident on thepixel per unit time.

in each of the solid-state image sensors disclosed by Japanese PatentLaid-Open No. 2004-363193 and Japanese Patent Laid-Open No. 2004-120391,a desired image is taken with a plurality of photoelectric conversionportions that are provided in each of pixels and having different levelsof sensitivity. In each of the devices, light is condensed by amicrolens provided on the surface of the pixel, whereby the light isguided to each of the photoelectric conversion portions. Hence, thequantity of light that is incident on each of the photoelectricconversion portions varies with the angle of incidence of the light onthe pixel. Therefore, the photoelectric conversion portions each receiveonly a portion of the light that is emitted from a specific part of theexit pupil of an image pickup lens used. Consequently, a blurred imageof an object that is out of focus may be distorted, resulting in adeterioration of image quality.

The present invention is to suppress the deterioration of image qualityby reducing the angular dependence of the sensitivity of each of aplurality of photoelectric conversion portions provided in each pixeland having different levels of sensitivity.

SUMMARY CF THE INVENTION

A solid-state image sensor according to a first aspect of the presentinvention includes pixels provided in a pixel area. The pixels eachinclude a first photoelectric conversion portion and a secondphotoelectric conversion portion that are provided in a substrate, thesecond photoelectric conversion portion having lower sensitivity thanthe first photoelectric conversion portion; a barrier region providedbetween the first photoelectric conversion portion and the secondphotoelectric conversion portion; a waveguide provided on alight-entrance side of the substrate and including a core and acladding; and a protective layer provided between the waveguide and thesubstrate. When a surface of the substrate is seen in a directionperpendicular to the substrate, a center of an exit face of the core ispositioned on a first-photoelectric-conversion-portion side with respectto a center of the barrier region at the surface of the substrate ineach of pixels that are provided in a central part of the pixel area. Astandard deviation of a distribution of refractive index of theprotective layer in a region directly below the exit face of the core is0.1 or smaller in an in-plane direction of the substrate.

An image pickup apparatus according to a second aspect of the presentinvention includes the solid-state image sensor according to the firstaspect that is provided in a housing.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary solid-stateimage sensor according to an embodiment of the present invention.

FIGS. 2A to 2C are diagrams illustrating an exemplary configuration of apixel included in the solid-state image sensor.

FIG. 3 is a graph illustrating the angular dependence of the sensitivityof each of first and second photoelectric conversion portions.

FIG. 4A is an exemplary plan view (XY-plane diagram) of the pixel seenfrom a light-entrance side.

FIG. 4B is an exemplary sectional view of the pixel that is taken alongline IVB-IVB illustrated in FIG. 4A.

FIG. 5 is a graph illustrating a relationship between the amount ofshift of a waveguide and the sensitivity ratio between the photoelectricconversion portions.

FIG. 6A is an exemplary plan view (XY-plane diagram) of another pixelseen from the light-entrance side.

FIG. 6B is an exemplary sectional view of the pixel that is taken alongline VIB-VIB illustrated in FIG. 6A.

FIGS. 7A to 7D are graphs illustrating the angular dependence of thesensitivity of each of the first and second photoelectric conversionportions with different amounts of shift of the waveguide.

FIG. 8A is a diagram schematically illustrating how a ray of light at anincident angle +θ is propagated in a case where asecond-photoelectric-conversion-portion-side end of an exit face of acore is positioned on a second-photoelectric-conversion-portion sidewith respect to a boundary between a barrier region and the secondphotoelectric conversion portion.

FIG. 8B is a diagram schematically illustrating now the ray of light atthe incident angle +θ is propagated in a case where thesecond-photoelectric-conversion-portion-side end of the exit face of thecore is positioned on a first-photoelectric-conversion-portion side withrespect to the boundary between the barrier region and the secondphotoelectric conversion portion.

FIGS. 9A to 9D are graphs illustrating the angular dependence of thesensitivity of each of the first and second photoelectric conversionportions with different values of the optical distance between the exitface of the core and the surface of a substrate.

FIG. 10A is a diagram illustrating an exemplary case where, seen in adirection perpendicular to the surface of the substrate, the center ofan entrance face of the core coincides with the center of the exit faceof the core.

FIG. 10B is a diagram illustrating an exemplary case where the center ofthe entrance face of the core is shifted toward thesecond-photoelectric-conversion-portion side with respect to the centerof the exit face of the core in an in-plane direction of the substrate.

FIG. 10C is a diagram illustrating an exemplary case where a part of thecore of the waveguide extends over an adjacent pixel.

FIG. 11A is a diagram illustrating an exemplary case where a microlensis provided at the extreme end on the light-entrance side of the pixel.

FIG. 11B is a diagram illustrating an exemplary case where a microlensis provided between the waveguide and the uppermost surface of thepixel.

FIG. 11C is a diagram illustrating an exemplary case where microlensesare provided at the extreme end on the light-entrance side of the pixeland between the waveguide and the uppermost surface of the pixel,respectively.

FIGS. 12A to 12D are diagrams illustrating microlenses of differentshapes that are applicable to the present invention.

FIGS. 13A and 13B are diagrams each illustrating the arrangement of thefirst photoelectric conversion portions and the second photoelectricconversion portions included in a plurality of pixels.

FIGS. 14A to 14C are graphs illustrating the dependence of the intensityof each of a high-sensitivity signal acquired by the first photoelectricconversion portion and a low-sensitivity signal acquired by the secondphotoelectric conversion portion upon the quantity of light that isincident on the pixel per unit time.

FIG. 15A is a diagram illustrating an exemplary case where the firstphotoelectric conversion portion and the second photoelectric conversionportion have the same capacity but different Z-direction depths.

FIG. 15B is a diagram illustrating an exemplary case where the firstphotoelectric conversion portion and the second photoelectric conversionportion have the same capacity and the same Z-direction depth.

FIG. 16 is a schematic diagram illustrating an image pickup apparatusincluding the solid-state image sensor according to the embodiment ofthe present invention.

FIG. 17A is a plan view of a pixel included in a related-art solid-stateimage sensor that is seen from a light-entrance side.

FIG. 17B is a diagram illustrating a sectional configuration of thepixel that is taken along line XVIIB-XVIIB illustrated in FIG. 17A.

FIG. 17C is an exemplary graph illustrating the dependence, upon theincident angle, of the sensitivity of each of the first and secondphotoelectric conversion portions included in a pixel according to therelated art.

FIG. 18A is a diagram illustrating how a ray of light that is incidenton the related-art pixel at an incident angle +θ is propagated.

FIG. 18B is a diagram illustrating how a ray of light that is incidenton the related-art pixel at an incident angle −θ is propagated.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the solid-state image sensor according to the presentinvention will now be described with reference to the drawings, whereinlike elements or elements having equivalent functions are denoted bylike reference numerals, and redundant description thereof is omitted.

FIG. 1 is a schematic diagram illustrating an exemplary solid-stateimage sensor 100 according to a general embodiment of the presentinvention. The solid-state image sensor 100 includes a pixel area 103 inwhich a plurality of pixels are provided, and an area in whichperipheral circuits 104 are provided.

Pixels

Pixels 101 refer to pixels provided in a central part 102 of the pixelarea 103. Herein, the pixels 101 provided in the central part 102 referto pixels whose centroids are positioned in the central part 102.

The central part 102 refers to an area within a predetermined distancefrom the center of the pixel area 103. The predetermined distance ispreferably ¼ of the length of the diagonal of the pixel area 103 orshorter, or more preferably 1/20 of the length of the diagonal of thepixel area 103 or shorter.

In the solid-state image sensor 100 illustrated in FIG. 1, the pixels101 provided in the central part 102 are in a 3-by-3 arrangement, forexample. The arrangement of the pixels 101 is not limited to such apattern. At least one pixel 101 only needs to be provided in the centralpart 102. Moreover, pixels having the same configuration as the pixel101 may be provided outside the central part 102.

FIGS. 2A to 2C are diagrams illustrating an exemplary configuration ofthe pixel 101 included in the solid-state image sensor 100. FIG. 2Aillustrates a layout of the pixel 101 at a surface of a substrate 120that is seen in a direction (Z direction) perpendicular to the substrate120. FIG. 2B is an XZ sectional view of the pixel 101 that is takenalong line IIB-IIB illustrated in FIG. 2A.

The pixel 101 includes, in order from a light-entrance side thereof, awaveguide 110 including a core 111 and a cladding 112, a protectivelayer 116, and the substrate 120. The substrate 120 includes a firstphotoelectric conversion portion 121, a second photoelectric conversionportion 122, and a barrier region 123 provided between the photoelectricconversion portions 121 and 122. Photoelectric Conversion Portions andBarrier Region.

The photoelectric conversion portions 121 and 122 are formed byproducing potential variations in the substrate 120 by ion implantationor the like. The substrate 120 is made of silicon or the like thatabsorbs light having a wavelength band to be detected. The barrierregion 123 has a potential barrier that suppresses the occurrence ofcharge crosstalk between the first photoelectric conversion portion 121and the second photoelectric conversion portion 122. As illustrated inFIG. 2C, the barrier region 123 corresponds to a region between thefirst photoelectric conversion portion 121 and the second photoelectricconversion portion 122 where the potential of the potential barrier is90% of the highest value or above, inclusive of the highest value.

The size of the potential barrier in the barrier region 123 may bedetermined with consideration for the permissible amount of chargecrosstalk between the first photoelectric conversion portion 121 and thesecond photoelectric conversion portion 122. To acquire pixel signalsfor the respective photoelectric conversion portions 121 and 122independently of each other, the potential barrier may be made high sothat the amount of charge crosstalk is reduced. More specifically, thepotential barrier in the barrier region 123 may have a height greaterthan or equal to the height of potential barriers (denoted by referencenumerals 128 and 129 in FIG. 2C) provided in other regions excluding thebarrier region 123 and surrounding the first photoelectric conversionportion 121 and the second photoelectric conversion portion 122. In sucha configuration, the crosstalk between the first photoelectricconversion portion 121 and the second photoelectric conversion portion122 can be reduced to approximately the same level as the crosstalkbetween pixels. However, if the permissible amount of charge crosstalkis large, the potential barrier in the barrier region 123 may be lowerthan the potential barriers provided in the other regions excluding thebarrier region 123 and surrounding the first photoelectric conversionportion 121 and the second photoelectric conversion portion 172.

To produce potential variations in the pixel 101 as illustrated in FIG.2C, a potential barrier may be formed by implanting ions into a regioncorresponding to the barrier region 123, not in regions corresponding tothe first photoelectric conversion portion 121 and the secondphotoelectric conversion portion 172. Alternatively, ions may beimplanted into both the photoelectric conversion portions 121 and 122and the barrier region 123. In that case, ions implanted into theregions corresponding to the photoelectric conversion portions 121 and122 and ions implanted into the barrier region 123 may have oppositeconductivity characteristics.

The first photoelectric conversion portion 121 and the secondphotoelectric conversion portion 122 do not necessarily need to bearranged side by side in the X direction as illustrated in FIGS. 2A and2B and may be arranged side by side in a direction that is at an anglelarger than 0° with respect to the X axis. Moreover, the shape of theopening in the surface of the substrate 120 at each of the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122 is not limited to a rectangle as illustrated inFIG. 2A and may be a circle, an ellipse, a polygon, or the like. Thecorners of the polygon may each be rounded in a manufacturing process.

Protective Layer

The protective layer 116 is provided for reducing the damage to thephotoelectric conversion portions 121 and 122 during a manufacturingprocess and for preventing impurities from entering the photoelectricconversion portions 121 and 122 from other members such as wiring lines125. In addition, the protective layer 116 may have another function,such as an anti-reflection function of suppressing the reflection oflight that is incident on the photoelectric conversion portions 121 and122 from the core 111. Moreover, the protective layer 116 may include aplurality of layers that are stacked in the direction perpendicular tothe surface of the substrate 120.

Waveguide

The waveguide 110 is formed of different materials that are combinedsuch that the core 111 has a higher refractive index than the cladding112. The materials for the core 111 and the cladding 112 can be selectedfrom inorganic materials such as silicon oxide, silicon nitride, siliconoxynitride, silicon carbide, and borophosphosilicate glass (BPSG), andorganic materials such as polymer and resin.

A broken line 111E illustrated in FIG. 2A defines an end facet of thecore 111 that is on the side of the substrate 120, i.e., an exit face.In FIG. 2A, the waveguide 110 is provided such that a center 113 of theexit face 111E of the core 111 is positioned on thefirst-photoelectric-conversion-portion side (−X side) with respect to acenter 124 of the barrier region 123. Herein, the center 124 of thebarrier region 123 at the surface of the substrate 120 refers to thecentroid of a plan-view shape, seen in the Z direction, of the barrierregion 123 at the surface of the substrate 120. Likewise, the center 113of the exit face 111E of the core 111 refers to the centroid of theplan-view shape, seen in the Z direction, of the exit face 111E of thecore 111. Hereinafter, for simplicity, the amount of shift of the center113 of the exit face 111E of the core 111 in the −X direction withrespect to the center 124 of the barrier region 123 at the surface ofthe substrate 120 is occasionally referred to as the amount of shift ofthe waveguide 110.

Light that is propagated in the waveguide 110 concentrates on the core111. Therefore, the quantity of light that is incident on the firstphotoelectric conversion portion 121 provided relatively near the center113 of the exit face 111E of the core 111 is larger than the quantity oflight that is incident on the second photoelectric conversion portion122 provided relatively far from the center 113 of the exit face 111E ofthe core 111. That is, shifting the center 113 of the exit face 111E ofthe core 111 toward the first-photoelectric-conversion-portion side withrespect to the center 124 of the barrier region 123 at the surface ofthe substrate 120 makes the sensitivity of the first photoelectricconversion portion 121 higher than the sensitivity of the secondphotoelectric conversion portion 122.

The cladding 112 is provided with wiring lines 125 that transmit drivingsignals for setting periods of exposure time of the respectivephotoelectric conversion portions 121 and 122 and read the signalsacquired by the photoelectric conversion portions 121 and 122. Thedriving signals that are transmitted from the peripheral circuits 104 tothe pixel 101 through the wiring lines 125 activate the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122 on the basis of respective desired periods ofexposure time.

SUMMARY

To summarize, in the pixel 101 included in the solid-state image sensor100 according to the general embodiment of the present invention, lightthat is incident on the pixel 101 is guided to each of the photoelectricconversion portions 121 and 122 by the waveguide 110 provided in such amanner as to be shifted with respect to the center 124 of the barrierregion 123 at the surface of the substrate 120. Employing such aconfiguration makes the angular dependence of the sensitivity of each ofa plurality of photoelectric conversion portions 121 and 122, which havedifferent levels of sensitivity, lower than that observed in therelated-art solid-state image sensor in which light that is incident onthe pixel is guided to each of the photoelectric conversion portions byusing a microlens. Comparison with the case of the related-artsolid-state image sensor will be given below.

Angular Dependence of Related-Art Solid-State Image Sensor

FIG. 17A is a diagram illustrating a layout of a pixel 1001 included inthe related-art solid-state image sensor, taken for comparison, at asurface (XY plane) of a substrate that is seen from a light-entranceside (seen in a direction perpendicular to the substrate). FIG. 17B is adiagram illustrating a configuration of section (XZ-plane) of the pixel1001 that is taken along line XVIIB-XVIIB illustrated in FIG. 17A. Thepixel 1001 illustrated in FIG. 17B includes no waveguide including acore and a cladding. The pixel 1001 is different from the pixel 101 ofthe sold-state image sensor 100 according to the general embodiment ofthe present invention illustrated in FIGS. 2A to 2C in that light thatis incident on the pixel 1001 is guided to each of first and secondphotoelectric conversion portions 1021 and 1022 by a microlens 1010.

FIGS. 18A and 18B illustrate how a ray of light that is incident on thepixel 1001 of the related-art solid-state image sensor is propagated.FIG. 18A illustrates how a ray of light traveling in the −Z directionand toward the +X side (at an incident angle +θ) is propagated. FIG. 18Billustrates how a ray of light traveling in the −Z direction. and towardthe −X. side (at an incident angle −θ) is propagated. As illustrated inFIGS. 18A and 18B, in accordance with the focusing characteristic of themicrolens 1010, the ray of light traveling in the −Z direction andtoward the +X side is selectively guided to the second photoelectricconversion portion 1022, whereas the ray of light traveling in the −Zdirection and toward the −X side is selectively guided to the firstphotoelectric conversion portion 1021. Consequently, the secondphotoelectric conversion portion 1022 is more sensitive to the ray oflight traveling in the −Z direction and toward the +X side, whereas thefirst photoelectric conversion portion 1021 is more sensitive to the rayof light traveling in the −Z direction and toward the −X side. That is,the sensitivity of the photoelectric conversion portions 1021 and 1022depends on the angle.

FIG. 17C is an exemplary graph illustrating the dependence, on theincident angle, of the sensitivity of each of the photoelectricconversion portions 1021 and 1022 included in the pixel 1001 providedwith the microlens 1010. The horizontal axis represents the incidentangle θ of a ray of light that is incident on the pixel 1001. A case ofθ=0° means that a ray of light parallel to the optical axis of themicrolens 1010 is incident on the pixel 1001.

As can be seen from FIG. 17C, the angular dependence of the sensitivityis particularly high in an angle range 1040 for both the firstphotoelectric conversion portion 1021 and the second photoelectricconversion portion 1022. If the angular dependence of the sensitivity ofthe photoelectric conversion portion is high, the photoelectricconversion portion receives only a ray of light emitted from a specificpart of the exit pupil of the image pickup lens used. Consequently, ablurred image of an object that is out of focus may be distorted,resulting in a deterioration of image quality.

Position of Waveguide and Angular Dependence

Now, a case where the microlens as the cause for the angular dependenceof the pixel is omitted will be discussed. The microlens provided on thelight-emission side of the pixel condenses light incident on the pixelfrom the outside and guides the light to the first photoelectricconversion portion and to the second photoelectric conversion portionprovided in the pixel. Hence, if the microlens is simply omitted,particularly, a ray of light that is obliquely incident on the pixeltravels straightly, without being condensed, and a portion thereofenters an adjacent pixel. Such a situation increases so-called crosstalkbetween pixels. Consequently, image quality may be deteriorated.

In contrast, the pixel 101 of the solid-state image sensor 100 accordingto the general embodiment of the present invention includes thewaveguide 110 whose center 113 of the exit face 111E of the core 111 isshifted toward the first-photoelectric-conversion-portion side withrespect to the center 124 of the barrier region 123 at the surface ofthe substrate 120, whereby light is guided to the photoelectricconversion portions 121 and 122, instead of condensing the light byusing the microlens. The light that enters the waveguide 110 is emittedafter being coupled with a plurality of waveguide modes. Therefore, theintensity distribution of the light at the exit face 111E of thewaveguide 110 is more even than that of the light condensed by themicrolens. Consequently, the angular dependence of the sensitivity ofeach of the photoelectric conversion portions 121 and 122 is reduced.Furthermore, a ray of light that is obliquely incident on the pixel 101is efficiently guided to the photoelectric conversion portions 121 and122 by the waveguide 110. Therefore, the crosstalk between pixels 101 isalso reduced.

FIG. 3 is a graph illustrating the angular dependence of the sensitivityof each of the photoelectric conversion portions 121 and 122 included.in the pixel 101 illustrated in FIGS. 2A to 2C. The horizontal axisrepresents the incident angle (θ), in the XZ plane, of a ray of lightthat is incident on the pixel 101. This applies to all graphsillustrating the angular dependence of the sensitivity of each of thephotoelectric conversion portions.

As is obvious from the comparison between FIG. 3 and FIG. 17C, thedependence of the sensitivity of each of the photoelectric conversionportions 121 and 122 upon the incident angle is far lower than that ofeach of the photoelectric conversion portions 1021 and 1022. Thephotoelectric conversion portions 121 and 122 each exhibit asubstantially uniform sensitivity characteristic, regardless of incidentangle. That is, the photoelectric conversion portions 121 and 122 cansubstantially evenly receive rays of light emitted from all over theexit pupil of the image pickup lens used. Consequently, the distortionof a blurred image of an object that is out of focus is reduced, andimage quality is improved.

Amount of Shift of Waveguide from Viewpoint of Sensitivity Ratio

The amount of shift of the center 113 of the exit face 111E of the core111 with respect to the center 124 of the barrier region 123 may bechanged in accordance with the required sensitivity ratio between thephotoelectric conversion portions 121 and 122. To make the sensitivityof the first photoelectric conversion portion 121 satisfactorily higherthan the sensitivity of the second photoelectric conversion portion 122,a second-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 may be positioned on thefirst-photoelectric-conversion-portion side with respect to a center 126of the second photoelectric conversion portion 122 as illustrated inFIGS. 4A and 4B. FIG. 4A is a plan view (XY-plane diagram) of the pixel101 seen from the light-entrance side (the pixel 101 seen in thedirection perpendicular to the substrate 120). FIG. 4B is a sectionalview of the pixel 101 that is taken along line IVB-IVB illustrated inFIG. 4A. The second-photoelectric-conversion-portion-side end 114 of theexit face 111E of the core 111 refers to a point of a plan-view shape,seen in the Z direction, of the exit face 111E of the core 111 that isat the extreme end on the second-photoelectric-conversion-portion side(a point where the X coordinate is largest in FIG. 4A). The center 126of the second photoelectric conversion portion 122 refers to thecentroid of the plan-view shape, seen in the Z direction, of the secondphotoelectric conversion portion 122 at the surface of the substrate120.

FIG. 5 illustrates a relationship between the amount of shift of thewaveguide 110 (represented by the horizontal axis of the graph) and thesensitivity ratio=(the sensitivity of the second photoelectricconversion portion 122)/(the sensitivity of the first photoelectricconversion portion 121) (represented by the vertical axis of the graph).The dash-dot line drawn in FIG. 5 represents a case where thesecond-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 coincides with the center 126 of the secondphotoelectric conversion portion 122. Accordingly, the zone on the rightside with respect to the dash-dot line corresponds to a case where theend 114 is on the −X side (the first-photoelectric-conversion-portionside) with respect to the center 126, which corresponds to thepositional relationship illustrated in FIGS. 4A and 4B. The zone on theleft side with respect to the dash-dot line corresponds to a case wherethe end 114 is on the +X side with respect to the center 126. As can heseen from FIG. 5, if the end 114 is on the +X side with respect to thecenter 126, the sensitivity ratio is higher than 50%, which means thereis substantially no difference in sensitivity between the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122. Hence, thesecond-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 may be positioned on thefirst-photoelectric-conversion-portion side with respect to the center126 of the second photoelectric conversion portion 122.

Amount of Shift of Waveguide from Viewpoint of Angular Dependence ofSensitivity

As illustrated in FIGS. 6A and 6B, thesecond-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 may be positioned on thefirst-photoelectric-conversion-portion side with respect to a boundary127 between the barrier region 123 and the second photoelectricconversion portion 122. Such a configuration can further reduce theangular dependence of the sensitivity of each of the first photoelectricconversion portion 121 and the second photoelectric conversion portion122.

Moreover, the second-photoelectric-conversion-portion-side end 114 ofthe exit face 111E of the core 111 may be positioned on thesecond-photoelectric-conversion-portion side (+X side) with respect to aboundary between the barrier region 123 and the first photoelectricconversion portion 121. If thesecond-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 is positioned on the −X side with respect to theboundary between the barrier region 123 and the first photoelectricconversion portion 121, substantially all rays of light emitted from thewaveguide 110 enter the first photoelectric conversion portion 121.Accordingly, the sensitivity of the second photoelectric conversionportion 122 becomes too low. Consequently, the quality of an imageacquired by the second photoelectric conversion portion 122 may bedeteriorated.

That is, the best position of thesecond-photoelectric-conversion-portion-side end 114 of the exit face111E of the core 111 is a position on thesecond-photoelectric-conversion-portion side with respect to theboundary between the barrier region 123 and the first photoelectricconversion portion 121 and on the first-photoelectric-conversion-portionside with respect to the boundary 127 between the barrier region 123 andthe second photoelectric conversion portion 122.

FIG. 6A is a plan view (XY-plane diagram) of the pixel 101 seen from thelight-entrance side (the pixel 101 seen in the direction perpendicularto the substrate 120). FIG. 6B is a sectional view of the pixel 101 thatis taken along line VIB-VIB illustrated in FIG. 6A. The boundary 127between the barrier region 123 and the second photoelectric conversionportion 122 refers to a point of a plan-view shape, seen in the Zdirection, of the second photoelectric conversion portion 122 that is atthe surface of the substrate 120 and at the extreme end on thefirst-photoelectric-conversion-portion side (a point where the Xcoordinate is smallest in FIGS. 6A and 6B).

FIGS. 7A to 7D are graphs illustrating the angular dependence of thesensitivity of each of the first photoelectric conversion portion 121and the second photoelectric conversion portion. 122 with differentamounts of shift of the waveguide 110. The amount of shift is reduced inorder of FIGS. 7A to 7D. FIGS. 7A and 7B each correspond to thearrangement illustrated in FIGS. 6A and 6B, in which the end 114 ispositioned on the −X side, i.e., thefirst-photoelectric-conversion-portion side, with respect to theboundary 127. FIGS. 7C and 7D each correspond to the arrangementillustrated in FIGS. 4A and 4B, in which the end 114 is positioned onthe +X side, i.e., the second-photoelectric-conversion-portion side,with respect to the boundary 127.

As can be seen from FIGS. 7A to 7D, the angular dependence of each ofthe photoelectric conversion portions 121 and 122 is lower in thearrangement corresponding to FIGS. 7A and 7B than in the arrangementcorresponding to FIGS. 7C and 7D. That is, if the end 114 is positionedon the first-photoelectric-conversion-portion side (−X side) withrespect to the boundary 127 as illustrated in FIGS. 6A and 6B, theangular dependence of the sensitivity of each of the first photoelectricconversion portion 121 and the second photoelectric conversion portion122 can be reduced further.

The reason for the lower angular dependence of the sensitivity of eachof the first photoelectric conversion portion 121 and the secondphotoelectric conversion portion 122 in the arrangement illustrated inFIGS. 6A and 6B is as follows.

FIG. 8A is a diagram schematically illustrating how a ray of light ispropagated in the case where the end 114 is positioned on thesecond-photoelectric-conversion-portion side (+X side) with respect tothe boundary 127. As illustrated in FIG. 8A, a ray 141 traveling in the−Z direction and toward the +X side within an angle range 140, that is,being incident on the pixel 101 at an angle +θ, is selectively coupledwith a plurality of specific waveguide modes 142 and exits from thewaveguide 110 into the second photoelectric conversion portion 122. Theray having entered the second photoelectric conversion portion 122 isconverted into electrons. Most of the electrons are accumulated in thesecond photoelectric conversion portion 122 with the effect of a strongdrift field of the second photoelectric conversion portion 122.Therefore, the sensitivity of the second photoelectric conversionportion 122 with respect to the ray 141 that is incident on the pixel101 at a specific angle (within the angle range 140 in FIGS. 7A to 7D)is increased, resulting in angular dependence.

FIG. 8B illustrates how the ray 141 that is incident on the pixel 101 atan angle +θ is propagated in the case where the end 114 is positioned onthe first-photoelectric-conversion-portion side (−X side) with respectto the boundary 127. In this case, as in the case illustrated in FIG.8A, the ray 141 incident on the pixel 101 at an angle +θ is selectivelycoupled with the plurality of specific waveguide modes 142, and the raythus coupled with the waveguide modes 142 exits from the waveguide 110.In this case, however, the ray having exited from the waveguide 110 doesnot enter the second photoelectric conversion portion 122 but enters thebarrier region 123. The ray having entered the barrier region 123 isconverted into photoelectrons in the barrier region 123. Thephotoelectrons are diffused in the substrate 120 and are accumulated ineither of the photoelectric conversion portions 121 and 122. Therefore,the angular dependence of the sensitivity of each of the photoelectricconversion portions 121 and 122 is lower than in the case illustrated inFIG. 8A.

That is, the angular dependence of the sensitivity of each of the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122 can further be made lower in the case where theend 114 is positioned on the −X side with respect to the boundary 127than in the case where the end 114 positioned on the +X side withrespect to the boundary 127.

As can be seen from the comparison between the case illustrated in FIGS.7C and 7D and the case illustrated in FIG. 17C, even if the end 114 ispositioned on the second-photoelectric-conversion-portion side withrespect to the boundary 127, the angular dependence the sensitivity ofeach of the first photoelectric conversion portion 121 and the secondphotoelectric conversion portion 122 is lower than that observed in therelated-art solid-state image sensor. This is because even the ray thathas been coupled with a plurality of specific waveguide modes 142exhibits a more even distribution of light intensity at the exit face111E of the waveguide 110 than in the case where light is condensed witha microlens.

Supplementary Explanation

The areas of the first photoelectric conversion portion 121 and thesecond photoelectric conversion portion 122 may be different asillustrated in FIGS. 2A to 2C or the same. Even if the photoelectricconversion portions 121 and 122 have the same aperture area, the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122 can have different levels of sensitivity, as longas the center 113 of the exit face 111E of the core 111 is shifted withrespect to the center 124 of the barrier region 123.

If the first photoelectric conversion portion 121 and the secondphotoelectric conversion portion 122 are arranged side by side in the XYplane and in a direction that is at an angle β (>0°) with respect to theX axis, the center 113 of the exit face 111E of the core 111 is shiftedalong the surface of the substrate 120 with respect to the center 124 ofthe barrier region 123 and in a direction that is at the angle β withrespect the X axis.

Protective Layer and Angular Dependence

Now, the thickness of the protective layer 116 will be described.

FIGS. 9A to 9D are graphs illustrating the angular dependence of thesensitivity of each of the first photoelectric conversion portion 121and the second photoelectric conversion portion 122 with differentvalues of optical distance L between the exit face 111E of the core 111and the surface of the substrate 120, i.e., with different thicknesses tof the protective layer 116. The optical distance L refers to an actualdistance t multiplied by a refractive index n of the protective layer116, which is expressed as L=n*t. If the protective layer 116 includes aplurality of different layers, the optical distance L is the sum ofvalues each being the actual distance of a corresponding one of thelayers that is multiplied by the refractive index of that layer.

The optical distance L increases in order of FIGS. 9A to 9D.

FIGS. 9A and 9B illustrate cases where L/λ is 0.5 and 1.5, respectively.The value L/λ is obtained by dividing the optical distance L by a centerwavelength k of light that is sensible by the first photoelectricconversion. portion 121 and the second photoelectric conversion portion122. FIGS. 9C and 9D illustrate cases where L/λ is 2.6 and 3.8,respectively. As can be seen from FIGS. 9A to 9D, the angular dependenceof the sensitivity of each of the first photoelectric conversion portion121 and the second photoelectric conversion portion 122 is lower in thecases illustrated in FIGS. 9A and 9B than in the cases illustrated inFIGS. 9C and 9D particularly in an angle range 140.

That is, the distance between the exit face 111E of the core 111 and alight-entrance-side surface of the substrate 120 may be short.Specifically, the optical distance L between the exit face 111E of thecore 111 and the surface of the substrate 120 may be set to twice thewavelength of light that is sensible by the photoelectric conversionportions 121 and 122 or shorter. Herein, the case where the light issensible by the photoelectric conversion portions 121 and 122 refers toa case where 5% or more of the light that is incident on the pixel 101is absorbed by a combination of the photoelectric conversion portions121 and 122.

The reason for the above is as follows. As described above, a ray thatis incident on the pixel 101 at an angle within the angle range 140 isselectively coupled with a plurality of specific waveguide modes 142.Then, the ray exits from the exit face 111E of the core 111 and iscoupled with a specific propagation mode in the protective layer 116.Hence, if the distance between the exit face 111E of the core 111 andthe surface of the substrate 120 is long, the ray exited from the exitface 111E of the core 111 is propagated within the protective layer 116before reaching the surface of the substrate 120. A portion of the raypropagated in such a manner enters the second photoelectric conversionportion 122 and is converted into electric charge. Most of such electriccharge is accumulated in the second photoelectric conversion portion122. Therefore, the sensitivity of the second photoelectric conversionportion 122 for the ray that is incident on the pixel 101 at a specificangle is increased, resulting in angular dependence of sensitivity.

Comparing FIGS. 9C and 9D with FIG. 17C, the angular dependence of thesensitivity is lower than that observed in the related-art solid-stateimage sensor even if the optical distance L between the exit face 111Eof the waveguide 110 and the surface of the substrate 120 is longer thantwice the wavelength of the light that is sensible by the photoelectricconversion portions 121 and 122. This is because, as described above,the intensity distribution, at the exit face 111E of the waveguide 110,of the light coupled with the plurality of specific waveguide modes 142is also more even than that of the light condensed by the microlens.

The distribution of refractive index of the protective layer 116 in aregion directly below the exit face 111E of the core 111 may be even inthe in-plane direction of the substrate 120 (in the XY plane). If therefractive index of the protective layer 116 varies in the in-planedirection, the propagation mode of the ray having exited from thewaveguide 110 with a nearly even distribution is changed by therefractive-index variation in the XY direction before the ray reachesthe surface of the substrate 120. Consequently, the angular dependenceof the sensitivity of each of the first photoelectric conversion portion121 and the second photoelectric conversion portion 122 is increased.The state where the distribution of refractive index is even refers to astate where the standard deviation of the distribution of refractiveindex is 0.1 or lower.

The distribution of refractive index of the protective layer 116 isobtained by measuring the refractive index of the protective layer 116in the region directly below the exit face 111E of the core 111 at fiveor more points on a line connecting the respective centroids of thefirst photoelectric conversion portion 121 and the second photoelectricconversion portion 122, including a point on the barrier region 123 anda point on the first photoelectric conversion portion 121. The points ofmeasurement are all at the same distance from the surface of thesubstrate 120 in the thickness direction. Specifically, if theprotective layer 116 includes a plurality of layers, the refractiveindex is measured within one layer.

The refractive index is measurable with an interferometer, anellipsometer, or the like. Alternatively, the composition of thematerial for the protective layer 116 may first be analyzed byFourier-transform infrared spectroscopy (FTIR), x-ray diffractometry(XRD), mass analysis, or the like, and the result may be converted intorefractive index.

Shape and Position of Waveguide

The shape of the waveguide 110 according to the general embodiment ofthe present invention is not limited to the shape illustrated in FIGS.2A and 2B, as long as the center 113 of the exit face 111E of the core111 is shifted toward the first-photoelectric-conversion-portion side(−X side) with respect to the center 124 of the barrier region 123 atthe surface of the substrate 120. For example, as illustrated in FIG.10A, seen in the direction (Z direction) perpendicular to the surface ofthe substrate 120, a center 115 of an entrance face of the core 111 maycoincide with the center 113 of the exit face 111E of the core 111.Alternatively, as illustrated in FIG. 10B, the center 115 of theentrance face of the core 111 may be positioned on thesecond-photoelectric-conversion-portion side (+X direction) with respectto the center 113 of the exit face 111E of the core 111 in the in-planedirection (XY direction) of the substrate 120. Moreover, as illustratedin FIG. 10C, a part of the core 111 of the waveguide 110 may extend overan adjacent pixel 101. Note that, to avoid the occurrence of crosstalkbetween adjacent pixels 101, afirst-photoelectric-conversion-portion-side end of the exit face 111E ofthe core 111 may be prevented from being positioned over an adjacentpixel 101. The first-photoelectric-conversion-portion-side end of theexit face 111E of the core 111 refers to a point of a plan-view shape,seen in the Z direction, of the exit face 111E of the core 111 that isat the extreme end on the first-photoelectric-conversion-portion side (apoint where the X coordinate is smallest in FIGS. 10A to 10C).

Furthermore, “the center 115 of the entrance face of the core 111”refers to the centroid of a plan-view shape, seen in the Z direction, ofthe entrance face of the core 111, as with “the center 113 of the exitface 111E of the core 111.”

The structure illustrated in FIG. 10A can be fabricated relativelyeasily. In the structure illustrated in FIG. 10B or 10C, it is easier toposition the second-photoelectric-conversion-portion-side end 114 of theexit face 111E of the core 111 on thefirst-photoelectric-conversion-portion side (−X side) with respect tothe boundary 127 between the barrier region 123 and the secondphotoelectric conversion portion 122. The waveguide 110 illustrated inFIG. 10B can be formed by performing etching after providing sacrificiallayers having different thicknesses or by performing multistage etchingwhile providing openings of different sizes.

The plan-view shape of the core 111 is not limited to the circular shapeas illustrated in FIG. 2A and may be an elliptical or polygonal shape.The corners of the polygonal shape may be rounded in a manufacturingprocess.

Microlens

A microlens may provided on the light-entrance side with respect to thewaveguide 110. If a microlens is provided, light that is incident on thepixel 101 can be efficiently guided to the wave guide 110. Note that, ifa microlens is provided, the angular dependence of the sensitivity of aregion between the photoelectric conversion portions 121 and 122 isincreased. Therefore, from the viewpoint of reducing the angulardependence, the microlens is not necessary. However, even if a microlensis provided, the angular dependence of each of the photoelectricconversion portions 121 and 122 is lower than that observed in therelated-art solid-state image sensor, because of the following reason.

If no waveguide is provided between the microlens and the photoelectricconversion portions as in the related-art solid-state image sensor, thefocusing effect of the microlens directly affects the angular dependenceof the sensitivity of the photoelectric conversion portions. Incontrast, if a waveguide is provided between the microlens and thephotoelectric conversion portions, light transmitted through themicrolens is incident on the entrance face of the waveguide. The lightthus incident on the entrance face of the waveguide is propagated in thewaveguide while being coupled with a plurality of waveguide modes, andis emitted toward the photoelectric conversion portions. Therefore, thedistribution of light intensity is more even at the exit face of thewaveguide than at the entrance face of the waveguide. That is, thedistribution of light intensity can be made more even than in the caseof the related-art sold-state image sensor in which light that isincident on the pixel is condensed on the photoelectric conversionportions while being transmitted only through the microlens. Hence,according to the general embodiment of the present invention, even if amicrolens is provided on the light-entrance side with respect to thewaveguide 110, the angular dependence of the photoelectric conversionportions 121 and 122 can be made lower than that observed in therelated-art solid-state image sensor.

FIG. 11A illustrates an exemplary case where a microlens 117 is providedon the uppermost surface of the pixel 101. FIG. 11B illustrates anexemplary case where a microlens 117 is provided between the waveguide110 and the uppermost surface of the pixel 101. In this case, themicrolens 117 is made of the same material as the core 111. FIG. 11Cillustrates an exemplary case where microlenses 117-1 and 117-2 areprovided on the uppermost surface of the pixel 101 and between thewaveguide 110 and the uppermost surface of the pixel 101, respectively.The microlenses 117 may each be made of an inorganic material such assilicon oxide, silicon nitride, silicon oxynitride, silicon carbide, orBPSG, or an organic material such as polymer or resin.

Microlens Having Different Levels of Refractive Power in DifferentDirections

As illustrated in FIGS. 12A to 12D, if a microlens 117 having differentlevels of refractive power in the X and Y directions is used, thesensitivity of the photoelectric conversion portions 121 and 122 can beincreased while the angular dependence of the sensitivity ratio isreduced. FIG. 12A illustrates an outline layout of a pixel 101 in the XYplane (a layout of the pixel 101 that is seen in the directionperpendicular to the substrate 120). FIGS. 12B to 12D each include anX-Z sectional diagram taken along line A-A. illustrated in FIG. 12A onthe left side and a Y-Z sectional diagram taken along line B-Billustrated in FIG. 12A on the right side.

If the refractive power of the microlens 117 in a first direction (alongline A-A) in which the line connecting the center of the firstphotoelectric conversion portion 121 and the center of the secondphotoelectric conversion portion 122 to each other extends is high, thedistribution of light intensity in the first direction at the entranceface of the waveguide 110 greatly depends on the angle of incidence ofthe light on the pixel 101 in the first direction. As thefirst-direction distribution of light intensity at the entrance face ofthe waveguide 110 becomes more uneven, the first-direction distributionof intensity of the light exiting from the waveguide 110 becomes moreuneven. Accordingly, the ratio between the quantities of light thatenters the respective photoelectric conversion portions 121 and 122changes. Thus, if the refractive power of the microlens in the firstdirection is high, the angular dependence of each of the photoelectricconversion portions 121 and 122 in the first direction is high.

In contrast, the refractive power of the microlens 117 in a seconddirection (along line B-B) perpendicular to the first direction at thesurface of the substrate 120 (in the XY plane) only affects thesecond-direction distribution of intensity of the light exiting from thewaveguide 110. The first photoelectric conversion portion 121 and thesecond photoelectric conversion portion 122 are provided side by side inthe first direction. Therefore, even if the intensity distribution isuneven in the second direction, the change in the ratio between thequantities of light that enters the respective photoelectric conversionportions 121 and 122 is small.

In view of the above, the refractive power of the microlens 117 in the Xdirection that greatly affects the angular dependence of sensitivity maybe reduced, whereas the refractive power of the microlens 117 in the Ydirection that little affects the angular dependence of sensitivity maybe increased, considering the sensitivity of the photoelectricconversion portions 121 and 122. That is, in the XY plane, the microlens117 may have a lower refractive power in the first direction in whichthe line connecting the center of the first photoelectric conversionportion 121 and the center of the second photoelectric conversionportion 122 are connected to each other than in the second directionthat is perpendicular to the first direction. Particularly, if the focalpoint of the microlens 117 in the second direction is defined near theentrance face of the core 111, the sensitivity of the photoelectricconversion portions 121 and 122 is maximized.

In each of the cases illustrated in FIGS. 12A to 12D, the firstdirection of the microlens 117 corresponds to the X direction, and thesecond direction of the microlens 117 corresponds to the Y direction.FIG. 12B illustrates a case where the radius of curvature or thelight-entrance face of the microlens 117 in the XZ section is largerthan that in the YZ section. FIG. 12C illustrates a case where therefractive power in the X direction is zero, that is, a case of acylindrical lens whose axis extends in the X direction. FIG. 12Dillustrates a case of a digital lens 118 which has a plurality ofmicroscopic structures each having a size of about 1/10 of thewavelength or small arranged such that the refractive power in the Xdirection is lower than the refractive power in the Y direction, wherebythe refractive-index distributions in the X direction and in the Ydirection are controlled independently of each other.

The above description all concerns a front-side-illuminated solid-stateimage sensor in which the wiring lines 125 are provided on the same sideof the substrate 120 as the waveguide 110. Alternatively, thesolid-state image sensor may be of a back-side-illuminated type in whichthe wiring lines 125 are provided on the other side of the substrate 120across from the waveguide 110. If the present invention is applied to aback-side-illuminated solid-state image sensor, the layout of thewaveguide 110 and the layout of the wiring lines 125 can be determinedindependently of each other. Therefore, the manufacturing process issimplified. Particularly, if a part of the waveguide 110 extends over anadjacent pixel 101 as illustrated in FIG. 10C, the solid-state imagesensor may be of the back-side-illuminated type, because the flexibilityin the layout of wiring lines 125 in the front-side-illuminated type islimited by the presence of the waveguide 110.

Arrangement of Pixels in Pixel Area

In the case where a plurality of pixels 101 are provided in the pixelarea 103 of the solid-state image sensor 100, the arrangement of thefirst photoelectric conversion portion 121 and the second photoelectricconversion portion 122 in a single pixel 101 may be the same for all ofthe plurality of pixels 101 or different between different pixels 101.

Note that, if a part of the waveguide 110 extends over an adjacent pixel101 as illustrated in FIG. 10C, an arrangement illustrated in FIG. 13Aor 13B may be employed, because the cores 111 of the waveguides 110 ofadjacent pixels 101 are prevented from interfering with each other.

FIG. 13A illustrates a case where the direction from the center of thefirst photoelectric conversion portion 121 toward the center of thesecond photoelectric conversion portion 122 In a single pixel 101 is the+X direction for all of the pixels 101. FIG. 13B illustrates a casewhere the direction from the center of the first photoelectricconversion portion 121 toward the center of the second photoelectricconversion portion 122 in a single pixel 101 is opposite between pixels101 that are adjacent to each other in the direction orthogonal to theline connecting the center of the first photoelectric conversion portion121 and the center of the second photoelectric conversion portion 122 toeach other. In FIGS. 13A and 13B, the first direction of each pixel 101is the same for all of the pixels 101, and the second direction istherefore the same for both of the adjacent pixels 101.

The direction from the center of the first photoelectric conversionportion 121 toward the center of the second photoelectric conversionportion 122 in a single pixel 101 may be different between differentpixels 101. That is, the direction may be the X direction in some pixels101, the Y direction in other pixels 101, and a direction that isoblique with respect to the X direction in vet other pixels 101.

First Embodiment

A first embodiment of the present invention will now be described withreference to FIG. 1 and FIGS. 2A to 2C. In the first embodiment, thesolid-state image sensor according to the present invention is used forforming an image with a wide dynamic range by acquiring ahigh-sensitivity signal and a low-sensitivity signal and combining thetwo together.

In the first embodiment, a high-sensitivity signal is acquired by thefirst photoelectric conversion portion 121, which receives a largerportion of the light, and a low-sensitivity signal is acquired by thesecond photoelectric conversion portion 122, which receives a smallerportion of the light. The photoelectric conversion portions 121 and 122are driven by receiving the respective signals transmitted from theperipheral circuits 104 through the wiring lines 125 and in such amanner as to be exposed to light for the same period of time.

The signals acquired by the respective photoelectric conversion portions121 and 122 are transferred to the peripheral circuits 104 through thewiring lines 125 and are output from the peripheral circuits 104 to anexternal device. The signals acquired by the photoelectric conversionportions 121 and 122 may be output from the peripheral circuits 104 asthey are. Alternatively, a high-sensitivity signal may be output if thequantity of light that is incident on the pixel 101 is smaller than athreshold, and a low-sensitivity signal may be output if the quantity oflight that is incident on the pixel 101 is larger than or equal to thethreshold. The threshold is set to a value smaller than a valuecorresponding to the signal intensity at which the high-sensitivitysignal is saturated and larger than a value corresponding to the signalintensity at which the low-sensitivity signal exhibits a desiredsignal-to-noise (SN) ratio.

Now, a method of widening the dynamic range by using thehigh-sensitivity signal and the low-sensitivity signal will bedescribed. FIGS. 14A to 14C are graphs illustrating the dependence ofthe intensity of each of a high-sensitivity signal 1031 and alow-sensitivity signal 1032 upon the quantity of light that is incidenton the pixel 101 per unit time. As illustrated in FIG. 14A, if theintensity of the high-sensitivity signal 1031 is lower than or equal toa first threshold 1051, the high-sensitivity signal 1031 is used. If theintensity of the low-sensitivity signal 1032 is higher than or equal toa second threshold 1052, the low-sensitivity signal 1032 is used. With acombination of the two signals 1031 and 1032, an image with a widedynamic range is acquired.

The first threshold 1051 is set to a signal intensity lower than thesignal intensity at which the high-sensitivity signal 1031 is saturated.The second threshold 1052 is set to a signal intensity at which the SNratio of the low-sensitivity signal 1032 exceeds a desired value. Hence,a quantity 1062 of light incident on the pixel 101 when the intensity ofthe low-sensitivity signal 1032 is equal to the second threshold 1052needs to be smaller than a quantity 1061 of light incident on the pixel101 when the intensity of the high-sensitivity signal 1031 is equal tothe first threshold 1051.

Here, let us consider the case of the related-art solid-state imagesensor illustrated in FIGS. 17A to 17C. As can be seen from FIG. 17C,the angular dependence of the sensitivity ratio between thephotoelectric conversion portions 1021 and 1022 is particularly high inthe angle range 1040. Therefore, the sensitivity ratio between thephotoelectric conversion portions 1021 and 1022 changes with the statesof the image pickup lens and the aperture that are used. Consequently,image quality may be deteriorated, or the dynamic range may be narrowed.The mechanism of such a situation is as follows.

FIG. 14B illustrates a case where the sensitivity ratio between thefirst photoelectric conversion portion 1021 and the second photoelectricconversion portion 1022 is high because the image pickup lens used has alarge f-number or the size of the aperture is reduced. This casecorresponds to, for example, a case where light at an angle within theangle range 1041 indicated in FIG. 17C is incident on the pixel 101.

In such a case, the quantity 1062 of light incident on the pixel 101when the intensity of the low-sensitivity signal 1032 is equal to thesecond threshold 1052 is larger than the quantity 1061 of light incidenton the pixel 101 when the intensity of the high-sensitivity signal 1031is equal to the first threshold 1051. Therefore, if the quantity oflight incident on the pixel 101 is within a range 1063, thehigh-sensitivity signal 1031 is saturated while the low-sensitivitysignal 1032 has an insufficient SN ratio. Hence, the quality of an imagecomposed may be deteriorated in the range 1063 corresponding to thepoint of switching between the high-sensitivity signal 1031 and thelow-sensitivity signal 1032. The signal is saturated when the quantityof light that is incident on the photoelectric conversion portionexceeds a level at which the amount of charge accumulated in thephotoelectric conversion portion reaches the maximum.

Now, let us consider a case where the sensitivity ratio between thefirst photoelectric conversion portion 1021 and the second photoelectricconversion portion 1022 is low. FIG. 14C illustrates the case where thesensitivity ratio between the first photoelectric conversion portion1021 and the second photoelectric conversion portion 1022 is low becausethe lens used has a small f-number or the aperture is fully opened. Thiscase corresponds to for example, a case where light within an anglerange 1042 indicated in FIG. 17C is incident on the pixel 101. In such acase, the quantity of light that is required for the acquisition of adesired SN ratio in the first photoelectric conversion portion 1021 islarger than in the case illustrated in FIG. 14A. In addition, the secondphotoelectric conversion portion 1022 is saturated with a smallerquantity of light than in the case illustrated in FIG. 14A.Consequently, the dynamic range of an image composed is narrowed by arange 1064 in which the intensity of the high-sensitivity signal 1031 islower than the second threshold 1052 and another range 1064 in which thelow-sensitivity signal 1032 is saturated.

As described above, if the angular dependence of the sensitivity ratiobetween the photoelectric conversion portions 1021 and 1022 is high, thequality of a resulting image may be deteriorated at the point, ofswitching between the two signals or the dynamic range may be narrowed,depending on the states of the lens and the aperture that are used forapplying light to the solid-state image sensor. In contrast, in thesolid-state image sensor 100 according to the first embodiment of thepresent invention, the angular dependence of the sensitivity of each ofthe photoelectric conversion portions 121 and 122 having differentlevels of sensitivity is low, as typically graphed in FIG. 3. Therefore,in addition to the above-mentioned reduction in the distortion of adefocused image, an image with an SN ratio that is higher than or equalto the desired level and with a wide dynamic range is acquired.

The required sensitivity ratio between the photoelectric conversionportions 121 and 122 is determined on the basis of the dynamic range orthe SN ratio that is required for an image to be composed. In the firstembodiment, the amount of shift of the center 113 of the exit face 111Eof the core 111 with respect to the center 124 of the barrier region 123may be determined on the basis of the dynamic range or the SN ratio thatis required for an image to be composed.

To widen the dynamic range by combining the two signals, the followingrelationship needs to be satisfied:

C1/S1>C2/S2   (Expression 1)

where S1 and C1 denote the sensitivity and the capacity, respectively,of the first photoelectric conversion portion 121 that acquires thehigh-sensitivity signal, and S2 and C2 denote the sensitivity and thecapacity, respectively, of the second photoelectric conversion portion122 that acquires the low-sensitivity signal.

The reason for this is as follows. If C1/S1≦C2/S2, the firstphotoelectric conversion portion 121 that acquires the high-sensitivitysignal is saturated with a quantity of light that is smaller than orequal to the quantity of light with which the second photoelectricconversion portion 122 that acquires the low-sensitivity signal issaturated. Moreover, C1/S1 may be twice the C2/S2 or larger. Note thatC1/S1 and C2/S2 are each the maximum quantity of light that may beaccumulated as charge in a corresponding one of the photoelectricconversion portions 121 and 122.

The ratio between the sensitivity S1 of the first photoelectricconversion portion 121 and the sensitivity S2 of the secondphotoelectric conversion portion 122 is changeable with the amount ofshift of the center 113 of the exit face 111E of the core 111 withrespect to the center 124 of the barrier region 123, as described above.

To increase the capacity C1 or C2 of the photoelectric conversionportion 121 or 122, the volume of the photoelectric conversion portion121 or 122 or the concentration of a dopant for forming thephotoelectric conversion portion 121 or 122 may be increased. Toincrease the volume of the photoelectric conversion portion 121 or 122,the opening of the photoelectric conversion portion 121 or 122 may bewidened by increasing the area of ion implantation, or the depth of thephotoelectric conversion portion 121 or 122 may be increased byimplanting ions deeply into the substrate 120. Note that, if thephotoelectric conversion portions 121 and 122 have the same depth andthe same dopant concentration, the photoelectric conversion portions 121and 122 can be formed under the same ion-implantation conditions,whereby the manufacturing process is simplified.

Even if the exposure time for the first photoelectric conversion portion121 and the exposure time for the second photoelectric conversionportion 122 are not the same, an image with a wide dynamic range can beacquired, as long as the amounts of charge that may be accumulated inthe respective photoelectric conversion portions 121 and 122 aredifferent. However, if the exposure time for the first photoelectricconversion portion 121 and the exposure time for the secondphotoelectric conversion portion 122 are different, the photoelectricconversion portions 121 and 122 particularly cause different levels ofmotion blur of the object. An image composed in such a manner may appearunnatural. Therefore, the exposure time for the first photoelectricconversion portion 121 and the exposure time for the secondphotoelectric conversion portion 122 are desired to be the same.

The pixel 101 may include three or more photoelectric conversionportions having different levels of sensitivity. If pixel signalsacquired by three or more photoelectric conversion portions havingdifferent levels of sensitivity are combined, the dynamic range of animage to be composed can be widened further.

Second Embodiment

A second embodiment of the present invention will now be described. Inthe second embodiment, the solid-state image sensor according to thepresent invention is used such that a plurality of photoelectricconversion portions having different levels of sensitivity are drivenfor different periods of exposure time, whereby an image taken with lowsensitivity and a long exposure time and an image taken with highsensitivity and a short exposure time are acquired simultaneously.

In general, the exposure time required for taking a smooth moving imageoften becomes longer than the exposure time required for taking a stillimage. Hereinafter, an image taken with low sensitivity and a longexposure time is regarded as a moving image, and an image taken withhigh sensitivity and a short exposure time is regarded as a still image.If the exposure time for the still image is longer than the exposuretime for the moving image, the photoelectric conversion portionsprovided for acquiring the moving image and the still image,respectively, only need to be interchanged with each other.

The second embodiment is different from the first embodiment in that theexposure time for the first photoelectric conversion portion 121 isshorter than the exposure time for the second photoelectric conversionportion 122. The first photoelectric conversion portion 121 acquires astill-image signal, and the second photoelectric conversion portion 122acquires a moving-image signal. The signals thus acquired are outputfrom the peripheral circuits 104 and are used for forming a still imageand a moving image, respectively.

Let us consider a case of the related-art solid-state image sensorillustrated in FIGS. 17A to 17C. As can be seen from FIG. 17C, theangular dependence of the sensitivity ratio between the photoelectricconversion portions 1021 and 1022 is particularly high in the anglerange 1040. Therefore, the sensitivity of each of the photoelectricconversion portions 1021 and 1022 changes with the states of the lensand the aperture that are used. Consequently, the quality of a resultingstill image and a resulting moving image may be deteriorated.

For example, if the lens used has a large f-number or the size of theaperture is reduced, the incident angle of the light that enters thepixel 1001 is within the angle range 1041 indicated in FIG. 17C. In theangle range 1041, the sensitivity of the first photoelectric conversionportion 1021 is high, and the sensitivity of the second photoelectricconversion portion 1022 is low. Consequently, the first photoelectricconversion portion 1021 is easily saturated, and overexposure tends tooccur in a resulting still image. In addition, the insufficiency in thesensitivity of the second photoelectric conversion portion 1022 tends tocause underexposure in a resulting moving image.

For example, if the lens used has a small f-number or the aperture isfully opened, the incident angle of the light that enters the pixel 1001is within the angle range 1040 indicated in FIG. 17C. In the angle range1040, the sensitivity of the first photoelectric conversion portion 1021is low, and the sensitivity of the second photoelectric conversionportion 1022 is high. Consequently, the sensitivity of the firstphotoelectric conversion portion 1021 becomes insufficient, andunderexposure tends to occur in a resulting still image. In addition,the second photoelectric conversion portion 1022 is easily saturated,and overexposure tends to occur in a resulting moving image.

As described above, if the angular dependence of the sensitivity of eachof the photoelectric conversion portions is high, the quality of aresulting still image and a resulting moving image may be deteriorated,depending on the states of the lens and the aperture that are used.

In contrast, in the solid-state image sensor 100 according to the secondembodiment of the present invention, the angular dependence of thesensitivity of each of the photoelectric conversion portions 121 and 122having different levels of sensitivity is low, as typically graphed inFIG. 3. Therefore, in addition to the above-mentioned reduction in thedistortion of a defocused image, a moving image and a still image thatare of high quality can be acquired simultaneously, regardless of thestates of the lens and the aperture that are used.

With the solid-state image sensor 100 according to the secondembodiment, different images, i.e., a still image and a moving image,can be formed from the still-image signal acquired by the firstphotoelectric conversion portion 121 and the moving-image signalacquired by the second photoelectric conversion portion 122,respectively. Therefore, the still-image signal and the moving-imagesignal may be generated with respective levels of intensity that are asclose to each other as possible and with respective dynamic ranges thatare as close to each other as possible.

Hence, the following relationships may also be satisfied:

S1×T1=S2×T2   (Expression 2)

C1/(S1×T1)=C2/(S2×T2)   (Expression 3)

where S1, C1, and T1 denote the sensitivity, the capacity, and theexposure time, respectively, of the first photoelectric conversionportion 121 that acquires the still-image signal, and S2, C2, and 12denote the sensitivity, the capacity, and the exposure time,respectively, of the second photoelectric conversion portion 122 thatacquires the moving-image signal.

Expression 2 defines a condition regarding the signal intensity.Expression 3 defines a condition regarding the dynamic range.

As described above, the ratio between S1 and S2 is controllable bychanging the amount of shift of the waveguide 110. As can be seen fromExpression 2, in the second embodiment, the ratio between S1 and S2 canbe determined by estimating the exposure time for each of a still imageand a moving image to be used. For example, if the exposure time for themoving image is 1/60 seconds and the exposure time for the still imageis 1/600 seconds, the pixel 101 is configured such that S1 is ten timesS2.

To satisfy Expressions 2 and 3 simultaneously, the capacity C1 of thefirst photoelectric conversion portion 121 and the capacity C2 of thesecond photoelectric conversion portion 122 may be the same. Herein, “tobe the same” implies that errors due to tolerances in the manufacturingprocess are permissible. Specifically, if the difference between thecapacity C1 of the first photoelectric conversion portion 121 and thecapacity C2 of the second photoelectric conversion portion 122 issmaller than 10% of the capacity C1 of the first photoelectricconversion portion 121, the capacities C1 and C2 are regarded as beingthe same.

As described above, the capacity of the photoelectric conversion portionis determined by the volume of the photoelectric conversion portion andthe concentration of the dopant for forming the photoelectric conversionportion. FIGS. 15A and 15B illustrate different cases in each of whichthe capacity of the first photoelectric conversion portion 121 and thecapacity of the second photoelectric conversion portion 122 are thesame. FIGS. 15A and 15B each include, on the left side, an XY plan viewof the pixel 101 that illustrates the layout at the surface of thesubstrate 120 seen in the Z direction (a plan view of the pixel 101 thatis seen in the direction perpendicular to the substrate 120) and, on theright side, an XZ sectional view of the pixel 101 that is taken alongline A-A illustrated in the XY plan view. In either case, the center 113of the exit face 111E of the core 111 is shifted toward thefirst-photoelectric-conversion-portion side (−X side) with respect tothe center 124 of the barrier region 123.

In the case illustrated in FIG. 15A, the X-direction length of the firstphotoelectric conversion portion 121 is longer than the X-directionlength of the second photoelectric conversion portion 122, and theZ-direction depth of the first photoelectric conversion portion 121 isgreater than the Z-direction depth of the second photoelectricconversion portion 122. In the case illustrated in FIG. 15B, the firstphotoelectric conversion portion 121 and the second photoelectricconversion portion 122 have the same X-direction length, the sameZ-direction depth, and the same dopant concentration. If thephotoelectric conversion portions 121 and 122 have the same depth andthe same dopant concentration, the photoelectric conversion portions 121and 122 can be formed under the same ion-implantation conditions,whereby the manufacturing process is simplified. Hence, in terms of themanufacturing process, the configuration illustrated in FIG. 15B issuperior.

The pixel 101 may include three or more photoelectric conversionportions. If the exposure time is varied among the three or morephotoelectric conversion portions and three or more images based on therespectively different periods of exposure time are acquiredsimultaneously, a plurality of images having different levels of blurcan be acquired. Moreover, photoelectric conversion portions to be usedmay be selected in accordance with the shutter speed that is set.According to Expressions 2 and 3, suitable characteristics of thephotoelectric conversion portions vary with the set shutter speed.Hence, if two of the three or more photoelectric conversion portions areselected, a still-image signal and a moving-image signal with closelevels of intensity and close dynamic ranges can be acquiredsimultaneously.

Signals acquired by a plurality of photoelectric conversion portionsexposed to light for the same period of time may be used for theacquisition of an image with a wide dynamic range, and signals acquiredby a plurality of photoelectric conversion portions exposed to light fordifferent periods of time may be used for the simultaneous acquisitionof a still image and a moving image.

Third Embodiment

FIG. 16 is a schematic diagram illustrating an image pickup apparatus190 including the solid-state image sensor 100 according to any of theembodiments of the present invention. The image pickup apparatus 190includes a housing 197 having a lens attaching portion 196 to which animage pickup lens 191 is attached, the solid-state image sensor 100, acontrol section 198 that controls the operation of the image pickupapparatus 190, and the image pickup lens 191 that takes light from theoutside into the housing 197. The image pickup lens 191 is attached tothe lens attaching portion 196 of the housing 197. The image pickup lens191 of the image pickup apparatus 190 may be removable from the housing197, i.e., interchangeable, or may be uninterchangeable. The controlsection 198 includes a central processing unit (CPU) 192, a transfercircuit 193, a signal processing unit 194, and a device driving circuit195.

The CPU 192 is a circuit that controls the transfer circuit 193, thesignal processing unit 194, and the device driving circuit 195. Thedevice driving circuit 195 is a circuit that drives the solid-stateimage sensor 100 in accordance with the signal from the CPU 192 andcontrols, for example, the periods of exposure time for the respectivephotoelectric conversion portions 121 and 122 provided in each of thepixels 101, and the timings of reading the signals acquired by thephotoelectric conversion portions 121 and 122. The transfer circuit 193stores the signals read from the solid-state image sensor 100 andtransfers the signals to the signal processing unit 194. The signalprocessing unit 194 processes the signals acquired through the transfercircuit 193 into an image.

The image pickup apparatus 190 is selectively operable in adynamic-range-widening mode in which the solid-state image sensor 100 isdriven in accordance with the first embodiment or in amoving-image-and-still-image-simultaneous-acquisition mode in which thesolid-state image sensor 100 is driven. in accordance with the secondembodiment. The mode is selectable by a user through an operation unit(not illustrated). The CPU 192 controls the associated circuits inaccordance with the mode selected.

If the dynamic-range-widening mode is selected, the solid-state imagesensor 100 is activated such that the first photoelectric conversionportion 121 and the second photoelectric conversion portion 122 areexposed to light for the same period of time and such that the firstphotoelectric conversion portion 121 having higher sensitivity acquiresa high-sensitivity signal and the second photoelectric conversionportion 122 having lower sensitivity acquires a low-sensitivity signal.If the quantity of light that is incident on the pixel 101 is lower thanthe threshold, the high-sensitivity signal is used. If the quantity oflight that is incident on the pixel 101 is higher than or equal to thethreshold, the low-sensitivity signal is used. With a combination of thetwo signals, an image with a wide dynamic range is formed.

If the moving-image-and-still-image-simultaneous-acquisition mode isselected, the solid-state image sensor 100 is activated such that theexposure time for the first photoelectric conversion portion 121 isshorter than the exposure time for the second photoelectric conversionportion 122. If the exposure time is set to a shorter value for thestill image than for the moving image for the purpose of, for example,shooting an object that is moving fast, a still-image signal is acquiredby the first photoelectric conversion portion 121 having highersensitivity while a moving-image signal is acquired by the secondphotoelectric conversion portion 122 having lower sensitivity. If theexposure time is set to a shorter value for the still image than for themoving image for the purpose of, for example, intentionally addingmotion blur, a moving-image signal is acquired by the firstphotoelectric conversion portion 121 having higher sensitivity while astill-image signal is acquired by the second photoelectric conversionportion 122 having lower sensitivity. The exposure time for the stillimage is determined by the user. The exposure time for the moving imageis set to about the value corresponding to the frame rate of thesolid-state image sensor 100. Thus, a still image and a moving image canbe formed simultaneously from the still-image signal and themoving-image signal acquired in the above manner.

The solid-state image sensor 100 of the image pickup apparatus 190 isnot limited to operate on the basis of only one of the first embodimentand the second embodiment. For example, the solid-state image sensor 100may have both the mode for acquiring an image with a wide dynamic rangeand the mode for simultaneously acquiring a moving image and a stillimage so that the mode is switched between the two in accordance withthe image to be acquired. In such a case, the levels of sensitivity andthe capacities of the first photoelectric conversion portion 121 and thesecond photoelectric conversion portion 122 need to satisfy at leastExpression 1. In addition, Expressions 2 and 3 may also be satisfied.

If both Expressions 2 and 3 are satisfied, the capacity of the firstphotoelectric conversion portion 121 and the capacity of the secondphotoelectric conversion portion 122 are the same. Therefore, Expression1 is naturally satisfied. Hence, in the solid-state image sensor 100having both the mode for acquiring an image with a wide dynamic rangeand the mode for simultaneously acquiring a moving image and a stillimage, the capacity of the first photoelectric conversion portion 121and the capacity of the second photoelectric conversion portion 122 maybe the same.

To summarize, the image pickup apparatus 190 according to the thirdembodiment is capable of acquiring an excellent image with a widedynamic range and is also capable of simultaneously acquiring a movingimage and a still image.

According to any of the embodiments of the present invention, asolid-state image sensor is provided in which the angular dependence ofthe sensitivity of each of a plurality of photoelectric conversionportions provided in each pixel and having different levels ofsensitivity is reduced, and the deterioration of image quality that mayoccur depending on the states of a camera lens and an aperture that areused is suppressed.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-024485, filed Feb. 10, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A solid-state image sensor comprising: pixelsprovided in a pixel area, the pixels each including a firstphotoelectric conversion portion and a second photoelectric conversionportion that are provided in a substrate, the second photoelectricconversion portion having lower sensitivity than the first photoelectricconversion portion; a barrier region provided between the firstphotoelectric conversion portion and the second photoelectric conversionportion; a waveguide provided on a light-entrance side of the substrateand including a core and a cladding; and a protective layer providedbetween the waveguide and the substrate, wherein, when a surface of thesubstrate is seen in a direction perpendicular to the substrate, acenter of an exit face of the core is positioned on afirst-photoelectric-conversion-portion side with respect to a center ofthe barrier region at the surface of the substrate in each of pixelsthat are provided in a central part of the pixel area, and wherein astandard deviation of a distribution of refractive index of theprotective layer in a region directly below the exit face of the core is0.1 or smaller in an in-plane direction of the substrate.
 2. Asolid-state image sensor according to claim 1, wherein, when the surfaceof the substrate is seen in the direction perpendicular to thesubstrate, a second-photoelectric-conversion-portion-side end of theexit face of the core is positioned on thefirst-photoelectric-conversion-portion side with respect to a center ofthe second photoelectric conversion portion.
 3. A solid-state imagesensor according to claim 1, wherein an optical distance between theexit face of the core and the surface of the substrate is, at maximum,twice a center wavelength of light that is sensible by the firstphotoelectric conversion portion and the second photoelectric conversionportion.
 4. A solid-state image sensor according to claim 1, wherein,when the surface of the substrate is seen in the direction perpendicularto the substrate, a center of an entrance face of the core coincideswith the center of the exit face of the core.
 5. A solid-state imagesensor according to claim l, further comprising a microlens provided onthe light-entrance side with respect to an entrance face of the waveguide.
 6. A solid-state image sensor according to claim 5, wherein arefractive power of the microlens in a first direction in which a lineconnecting a center of the first photoelectric conversion portion and acenter of the second photoelectric conversion portion to each otherextends is lower than a refractive power of the microlens in a seconddirection perpendicular to the first direction in a plane parallel tothe surface of the substrate.
 7. A solid-state image sensor according toclaim 1, further comprising a wiring line that transmits signalsacquired by the first photoelectric conversion portion and the secondphotoelectric conversion portion, the wiring line being provided on aside of the substrate that is opposite the side on which the waveguideis provided.
 8. A solid-state image sensor according to claim 1, whereinthe barrier region has a potential barrier whose height is greater thanor equal to a height of a potential barrier of a region excluding thebarrier region and surrounding the first photoelectric conversionportion and the second photoelectric conversion portion.
 9. Asolid-state image sensor according to claim 1, wherein the central partis an area within a distance from a center of the pixel area of ¼ of alength of a diagonal of the pixel area.
 10. A solid-state image sensoraccording to claim 1, wherein a direction from the center of the firstphotoelectric conversion portion toward the center of the secondphotoelectric conversion portion in a single pixel is opposite betweenpixels that are adjacent to each other in a direction orthogonal to theline connecting the center of the first photoelectric conversion portionand the center of the second photoelectric conversion portion to eachother at the surface of the substrate.
 11. A solid-state image sensoraccording to claim 1, wherein a signal acquired by the firstphotoelectric conversion portion is output if a quantity of light thatis incident on the pixel is smaller than a predetermined threshold, anda signal acquired by the second photoelectric conversion portion isoutput if the quantity of light that is incident on the pixel is largerthan the predetermined threshold.
 12. A solid-state image sensoraccording to claim 1, wherein exposure time for the first photoelectricconversion portion is shorter than exposure time for the secondphotoelectric conversion portion.
 13. A solid-state image sensoraccording to claim 12, wherein the following relationships aresatisfied:S1×T1=S2×T2, andC1/(S1×T1)=C2/(S2×T2), where S1, C1, and T1 denote sensitivity,capacity, and exposure time, respectively, of the first photoelectricconversion portion, and S2, C2, and T2 denote sensitivity, capacity, andexposure time, respectively, of the second photoelectric conversionportion.
 14. An image pickup apparatus comprising: the solid-state imagesensor according to claim 1 that is provided in a housing.
 15. An imagepickup apparatus according to claim 14, wherein an image is formed byusing a signal acquired by the first photoelectric conversion portion ifa quantity of light that is incident on the pixel is smaller than apredetermined threshold and by using a signal acquired by the secondphotoelectric conversion portion if the quantity of light that isincident on the pixel is larger than the threshold.